A control device of a battery of a marine vessel that ensures electric power for starting an engine with priority is provided. A first value indicating either a voltage or a power storage amount of a main battery that supplies electric power for starting the engine is acquired. In a case where the first value that has been acquired is smaller than a first threshold value, at least one of a prohibition of a transition to a motor drive mode (S104) in which a propeller is driven by using at least a motor regardless of an operation mode that is designated and a prohibition of charging of a load battery is performed, and the engine is caused to rotate to charge the main battery.